There is known a communication system including a plurality of switches that each transfer a packet, and a control unit that controls each switch, in the communication system of which, the control unit sets flow entry information in each switch and the switch transfers a received packet in accordance with the flow entry information. The flow entry information provides for the type of processing (e.g., transferring, discarding, updating, or others) that the received packet is to undergo, depending on a header of the packet. Such a communication system requires a control channel used for the control unit to control the switch. For example, if the switch receives a packet not defined in the flow entry information, the switch notifies this to the control unit and then the control unit transmits appropriate flow entry information to the switch according to the particular packet. The control channel is used for purposes such as the notification from the switch to the control unit and the transmission of flow entry information from the control unit to the switch. The protocol used for the control unit to control the switch is called “OpenFlow”. The switch can be considered as a packet transfer unit that transfers the packet. The control channel is equivalent to a “secure channel” in the OpenFlow protocol. Also, the control unit is called the “controller” in OpenFlow.
Specifications of OpenFlow are described in Non-Patent Document 1. It is provided for in the specifications of OpenFlow that the control unit and the switch should use SSL (Secure Socket Layer) to communicate in TCP (Transmission Control Protocol) of port number 6633. Accordingly, the communication systems to which the OpenFlow protocol is applied commonly include two independent communications networks. One is a communications network with a plurality of switches for transferring packets in order, and the other is a communications network that operates as the control channel between the control unit and each switch. FIG. 27 is an explanatory diagram that shows an example of a general communication system to which the OpenFlow protocol is applied. Each of the switches 92 shown in FIG. 27 uses a packet transfer communications network 94 to transfer the received packet to other switches in accordance with flow entry information. In addition, a control communications network 93 to serve as the control channel is provided independently of the packet transfer communications network 94, and a control unit 91 controls each switch 92 via the control communications network 93. The control communications network 93 uses a routing protocol such as STP (Spanning Tree Protocol), TCP/IP (Transmission Control Protocol/Internet Protocol), RIP (Routing Information Protocol), or OSPF (Open Shortest Path First).
Furthermore, in the communication system shown by way of example in FIG. 27, when a topology of the switches is changed, each switch exchanges control frames in the communication system and recognizes a new topology. Examples of a control frame for recognizing the topology include, for example, BPDU (Bridge Protocol Data Unit) in STP, and others.